PID CONTROLLER WITH DIRECT ACTION (NEGATIVE GAIN)

• Picture of the frontpanel of the simulator
• What is needed to run the the simulator? Read to get most recent information!
• Tips for using the simulator.
• The simulator: pid_reverse_direct.exe (click to download). The simulator runs immediately after the download by clicking Open in the download window. Alternatively, you can first save a copy of the exe-file on any directory (folder) on your PC and then run the exe-file, which starts the simulator.

Description of the simulated system

In this simulator a level control system for a liquid tank is simulated. The control signal controls the mass outflow from the tank. The user can select the sign of the controller gain.

Aim

The aim of this simulator is to observe the consequences of selecting wrong sign of the controller gain.

Motivation

Commercial controller give the user the option to select the controller gain according to the process gain. Selecting wrong sign has dramatic concequences for the stability of the control loop.

Tasks

1. Explain (without simulating) that the process has negative gain.
2. Confirm by running a simulation that the process has negative gain. (Tip: Set the controller in manual mode.)
3. Normally the controller gain Kp in PI(D) controllers have positive sign. Thus, the controller has so-called reverse action (since an increase of the process measurement value decreases the control signal). Run a simulation. What happens to the stability of the control system if Kp is positive? You may set Kp = 2.5 and Ti = 4.
4. Ensure that the controller has negative gain, i.e. the controller has direct action. Run a simulation. What is the stability of the control system?

[KYBSIM] [TechTeach]

Updated September 11, 2004. Developed by Finn Haugen. E-mail: finn@techteach.no.